Heterostructure field-effect transistors (HFETs), especially those made of wide bandgap semiconductor materials such as AlGaInN, offer tremendous advantages for high-power, high-frequency electronics over many other transistor types (e.g., Si based MOSFETs and bipolar transistors, GaAs and other III-V compound based MESFETs, etc). These advantages are related to very high two-dimensional (2D) electron gas densities at the heterointerface, high operating temperatures, and chemical inertness and radiation hardness of III-N and other wide bandgap materials. Furthermore, the insulated gate HFET modification (MOSHFET) offers additional advantages in terms of extremely low gate leakage currents, higher channel currents, and improved stability and reliability.
The majority of the HFETs and MOSHFETs are so-called depletion-mode (also referred to as “normally-on”) devices. This implies that when no or zero voltage is applied at the gate electrode, the transistor is in its “on” state, a conducting channel exists between the source and drain electrodes, and significant current is delivered into the load. Gate voltage needs to be applied to this type of transistor to turn it “off”, deplete the channel, and make the drain current close to zero. For n-channel HFETs, the required gate voltage is negative.
Depletion mode devices have several significant drawbacks for practical applications. For example, depletion mode devices consume power when no gate voltage (i.e., input signal) is applied. This decreases the overall system power efficiency, which has a negative impact on integrated digital electronics and power electronic circuits utilizing HFETs. Further, HFET circuits require dual-polarity voltage supplies: positive for drain biasing and negative for gate biasing.
Enhancement-mode (E-mode) devices have their channel in the “off” state when the gate voltage is zero; thus they are free from the above drawbacks. However, such a “normally-off” performance is hardly achievable in a regular GaN-based HFET design due to the presence of 2D electron gas in the HFETs induced by polarization doping. Such polarization doping is caused by the differences between piezoelectric and spontaneous polarizations in the wide band gap barrier layer material (typically AlGaN) and in the channel.
Several approaches to creating enhancement mode HFETs have been described in the related art, requiring either plasma treatment or selective barrier etching under the gate, or very thin barriers to reduce the 2D electron gas density at zero gate bias. These approaches, however, lead to significant technological complications, high access region resistances, and resulting performance limitations.
A GaAs-based E-mode HFET structure using selective barrier etching is shown in FIG. 1. This technology is much more complicated than that of regular HFETs as it requires precise etched thickness control. As applied to III-Nitride materials, etching adversely affects the HFET performance; the devices with thin barriers have higher access resistances and lower peak currents. As an example, as shown in FIG. 2, the peak currents for a related art AlGaN/GaN E-mode device were 0.25 A/mm, whereas depletion mode AlGaN/GaN HFETs have peak current in excess of 1 A/mm.